1. Field of the Invention
The present invention relates to a polymer film and a preparation method of a polymer solution, especially to a polymer film adequately used for a retardation filter, a polarizing filter and an image displaying device, and a preparation method of a polymer solution used for producing the polymer film in a solution casting method.
2. Description Related to the Prior Art
For a photosensitive material of silver halide, a birefringence filter, a polarizing plate and an image display device is used a polymer represented by cellulose ester, polyester, polycarbonate, cycloolefine polymer, vinylpolymer and polyimide. A polymer film produced from the polymers is usually produced in a solution casting method or a melt-casing method. For example, Japanese Patent Laid-Open Publication No. 2003-236915 disclose a method in which cycloolefin co-polymer having a relatively low glass transition point Tg and a relatively low melting point Tm is used to produce a film for optical use in a melt extruding method. Otherwise, in the solution casting method, a polymer solution in which polymer is dissolved into a solvent is cast on a substrate continuously conveyed, and when having a self-supporting properties, the cast polymer is peeled from the substrate. Then the solvent is evaporated. In comparison to the melt extruding method, the polymer film which is more excellent in planarity and uniformity can be produced, and therefore widely adopted for an optical use. A representative example of the film for optical use as a production in the solution casting method is a cellulose ester film which is used as a silver halide photosensitive material and a protective film of a polarizing filter.
As a solvent for preparing the cellulose ester solution used for producing a cellulose ester film, chlorine type hydrocarbon such as dichloromethane is used. However, the research for using other solvents type for cellulose ester than the chlorine type organic solvent proceeds. For example, organic solvents having a dissolubility to cellulose ester, especially cellulose triacetate are acetone (boiling point 56° C.), methyl acetate (boiling point 56° C.), tetrahydrofurane (boiling point 65° C.), 1,3-dioxolane (boiling point 75° C.), 1,4-dioxane (boiling point 101° C.) and the like. However, since these organic solvents don't have enough dissolubility for practice actually, a cool-dissolving method in which cellulose ester is dissolved in a low temperature is combined. For example, in the publication “Makromol. Chem., 1971, Vol. 143, P105”, cellulose triacetate (acetylation degree in the range of 60.1% to 61.3%) is cooled in acetone to have a temperature in the range of −80° C. to −70° C., and thereafter the heating is made such that a dilute solution in which the content of cellulose acylate is in the range of 0.5.% to 5% may be obtained. Further, the publication in “Journal of Textile Machinary Society, 1981, Vol 34, p. 57-61” proposes a fiber spinning technique with use of the cool-dissolving method. Further, in Japanese Patent Laid-Open Publications No. 9-95538, 9-95544 (pages 4-7), 9-95557 (pages 4-8), non-chlorine type organic solvents are used, and cellulose acylate is dissolved to the organic solvents for preparing the solution.
Recently the demands of the image display device is large. The liquid crystal display includes the retardation filter and the protective film for the polarizing filter. The improvement of the compensatory character of the birefringence is required in accordance with widening the view angle. However, in the film above described, a wavelength diffusion value (ΔRe) of an in-plane retardation (a retardation in an in-plane direction) and a wavelength diffusion value (ΔRth) of a thickness retardation (or a retardation in a thickness direction) has the same sign. Accordingly, the optical compensation method or the compensatory character are restricted. Therefore a method in which the wavelength relativity of each retardation is independently controlled is necessary.
However, in order to obtain the image displaying device in which the wavelength relativity is satisfied in accordance with widening the view angle, it is necessaty to control the birefringence of the polymer film used, and the control method has a limitation so far as being the stretch by a tenter in the prior solution casting method, and a mechanical treatment. Further, in the solution casting method, the birefringence control method to supply the polymer film with the optical compensatory character enough to satisfying the above wavelength relativity, although the film having more excellent optical property than the melt extruding method can be produced. Otherwise, according to a preparation method of the polymer solution used in the solution casting method, the supposition of improvement of the solubility is made. However, there are only some examples of the preparation method in view of optical properties of the birefringence of the produced film.